The present invention relates to a radial ply tire for motorcycles, and more particularly to an improvement in the breaker reinforcement, which improves the steering stability at high running speed, and lessens the ply separation at the breaker edges.
According to recent developments of motorcycles, the running speed has increased. Therefore, motorcycle tires are also required to be improved in various characteristics under high speed running.
In general, for the carcass of a motorcycle tire, there has been employed the cross-ply structure, in which the cords of each ply are arranged at 25 to 60 degrees to the circumferential direction of the tire so as to cross with the cords of the next ply. This is because the requirements for motorcyle tires are radically different from those for four-wheel vehicles such as passenger cars, particularly with respect to tire behavior during cornering.
That is, a motorcycle, during cornering, is tilted at a large bank angle with respect to a plane perpendicular to the road surface. Therefore, it is necessary for the tire to have such characteristics as being able to produce a stably camber trust (a force parallel with the road surface in the same direction as the bank) to balance with the centrifugal force acting upon the motorcycle.
Accordingly, the motorcycle tires must be transversely stiffened, and this used to be done by employing a cross-ply type carcass as described above so as to provide and maintain the camber trust. Thus a radial ply structure, inferior to the cross ply structure in transverse stiffness has seldom been used.
The cross-ply tire, however, has such drawbacks that the resistance to wear is poor, and transverse vibrations known as the weave phenomenon of the motorcycle frame are caused during high speed running. Accordingly, as countermeasure for such problems, increasing the stiffness of the tire as a whole by, for examples, arranging the carcass cord angle, increasing the number of carcass plies, and/or the like has been employed, but this could not completely remove the drawbacks of the cross ply structure.
Recently, therefore, it has been proposed to use a radial ply carcass for motorcycle tires to try to solve the above-mentioned drawbacks of the conventional cross-ply construction.
On the other hand, it has been known that when the radial ply carcass is applied to the motorcycle tires, which needs breaker reinforcement, separation of the edge portions of the breaker ply from the surrounding rubber frequently occurs unlike passenger car tires. This is explained as follows:
As shown typically in FIG. 2(a), in motorcycle tires, the radius of curvature of the tread is small, that is, the tread is round, and the camber value H/L is large. On the other hand, the breaker ply is extended across the whole width of the tread. As a result, the tension of the breaker cords in the tread center region becomes different from that in the tread edge regions, and the stress concentrates in the breaker edges. Further, the tread is subjected to repeated deformation from the tire unloaded or free state of FIG. 2(a) to the loaded or ground contacting state of FIG. 2(b) and vice versa. And when the tread is deformed into the ground contacting state from the free state, the radius of curvature of the inner surface of the tire shoulder is decreased to R2 (FIG. 2(b)) from R1 (FIG. 2(a)). As a result, shear stress is generated on the breaker edge portions because they are located in such regions.
It has also been proposed to use low elasticity modulus cords, such as nylon cords, in place of steel cords generally used for the breaker cords of radial tires. In this case, however, due to the resultant low rigidity of breaker, the steering stability becomes poor, and the growth of the tire when inflated becomes large.